M4040 [2006-N] Nissan Engineering Standard
NES
Specification − Zinc Plating and Chromate Films on Iron and Steel
1. SCOPE This Standard describes electroplated coatings of zinc plating and chromate films hereinafter referred to as plating that are applied to iron and steel for corrosion protection and appearance. Remarks: 1. This Standard applies to the significant surface that is defined as the surface for which the plating is most important. Corrosion protective qualities, for parts other than the significant surface, shall be determined by agreement between the parties concerned. 2. The significant surface shall be as specified by the drawing or the inspection standard. 3. The material specified in this NES shall satisfy NES M 0301 “Substance Use Restrictions”. 2. DEFINITION OF TERMS Major terms used in this Standard shall be defined as follows: (1) Yellow chromate Chromate film, intended for corrosion protection, is yellow. Therefore, the corrosion protective chromate film is called yellow chromate. Its color tone may change depending on the composition of chromium chromate (xCr 2O3/yCrO3/zH2O), a major component of the film. (2) Bright (clear) chromate Chromate processing creates a corrosion protective film on a zinc plating while also acting as a chemical polishing agent. This chemical polishing action is used to give gloss to plating. Methods of treatment are classified into chemical polishing and alkali liquid treatment. In the chemical polishing method, the creation of corrosion protective film is rather suppressed. In the alkali treating method a corrosion protective film is produced and then treated by the alkali solution to have a glossy surface. The corrosion protective performance of this bright chromate is inferior to the yellow chromate. (3) Black chromate After plating, the chromate film is form ed with a processing solution containing chromic acid and an additive of black color treatment chemical (silver nitrate, etc.). Black chromate does not provide any significant abrasion resistance. (4) Olive (green) chromate After plating, a chromate processing solution containing special anions is used. A thick film-type coating can be obtained with good resistance to corrosion. The coating is also called green chromate. (5) Interference fringe on the surface With a thin chromate film, a transparent, invisible thin film remains on the plating. When white light illuminates this surface, rainbow color is created as a result of interference between reflected light from the surfaces of the coating film and plating. This fringe pattern is called interference fringe. (6) Dispersion of color on the surface The color tone of the yellow chromate varies depending on the composition of the coating. The composition may fluctuate due to variations in the chromate processing solution and operating conditions. Therefore, even among the same lot of parts, the color tone of each part often becomes different. Even on the same surface, uniform color tone cannot be obtained very easily; therefore, there may be color irregularities. (7) White corrosion products The corrosion products of zinc consist mainly of white compounds such as zinc carbonate. When zinc is exposed to a corrosive environment while the chromate film is broken, such corrosion products (zinc rust) are created.
Normative References:
NES M 0140-2001
Methods of Salt Spray Testing
NES M 2023-2003
Double Wall Welded Steel Tubes
NES M 0301-2005-1 Substance Use Restrictions References:
JIS D 0201-1995
General rules of electroplating for automobile parts
JIS H 8610-1999
Electroplated coatings of zinc on iron and steel
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M4040 [2006-N] (8) Trivalent clear chromate The film of the trivalent clear chromate is a film intending to be free from hexavalent chromium which is generated by processing with liquid of trivalent chrome and silica as main components after plating. The color tone is similar to that of the bright chromate. (9) Trivalent black chromate Trivalent black chromate film is a black chromate that is free of hexavalent chromium. After the plating, the film is formed by a treatm ent liquid that consists of trivalent chrome and silica doped with black process chemicals (oxide, such as iron, cobalt, etc.). 3. TYPES AND GRADES The types and grades of plating shall be classified by the methods of surface treatment and plating thicknesses as shown in Table 1. Table 1 Type
Type 1
Type 2
Type 3
Type 4
Type 5
Type 6
Yellow chromate
Bright chromate
Black chromate
Olive chromate
Trivalent clear chromate
Trivalent black chromate
Types
Rating
Symbol
Plating 1 thickness ( ) µm
1
MFZn 3C
3
Slight corrosion protection parts
2
MFZn 5C
5
General screw parts and interior parts
3
MFZn 8C
8
Exterior parts and screw parts
4
MFZn 13C
13
High corrosion protection parts
1
MFZn 3B
3
Slight corrosion protection parts
2
MFZn 5B
5
General screw parts
3
MFZn 8B
8
Interior parts
4
MFZn 13B
13
Exterior parts
1
MFZn 3K
3
Slight corrosion protection parts
2
MFZn 5K
5
General screw parts and interior parts
3
MFZn 8K
8
Exterior parts
4
MFZn 13K
13
High corrosion protection parts
1
MFZn 3G
3
High corrosion protection general parts
2
MFZn 5G
5
High corrosion protection general screw parts and interior parts
3
MFZn 8G
8
High corrosion protection exterior parts and screw parts
4
MFZn 13G
13
High corrosion protection parts
6
MFZn 25G
25
Hydraulic brake piping ( )
1
MFZn 3TC
3
Slight corrosion protection parts
2
MFZn 5TC
5
General screw part s and int erior parts
3
MFZn 8TC
8
Exterior parts and screw parts
4
MFZn 13TC
13
High corrosion prot ection parts
1
MFZn 3TK
3
Slight corrosion protection parts
2
MFZn 5TK
5
General screw part s and int erior parts
3
MFZn 8TK
8
Exterior parts
4
MFZn 13TK
13
High corrosion protection parts
Example of application
Reference
2
Substitute material to hexavalent chromium in Types 1 and 2 Substitute material for hexavalent chromium in Type 3
1
Note: ( ) The plating thicknesses show minimum values. 2 ( ) See BUD-Z6, NES M 2023 (Double W elded Steel Tubes).
Remarks: 1. Chromate treatment shall generally apply to plating. 2. Type 1 is mainly intended for corrosion protection, applying yellow chromate treatment after plating. 3. Type 2 means the bright chromate treatment after plating, but its corrosion resistance is inferior to Type 1. 4. Type 3 and Type 6 are used mainly for decorative purposes. This type shall not be applied to parts touched by hands usually. 5. Type 4 means olive chromate treatment after plating and is used for high corrosion resistance. 6. Type 5 is mainly intended for corrosion protection, applying trivalent clear chromate treatment after plating.
2
M4040 [2006-N] 7. The interior parts mean those mounted in the body of a car and not subjected to direct or indirect wind or rain. 8. The following cases shall be exempt from application: (1) Parts coated with grease in use and not requiring protection from corrosion. (2) Parts not visible and not requiring plating but actually plated in the manufacturing process. (3) Parts treated for rust prevention by another treatment due to a machining reason such as caulking. 9. The major constituents of the hexavalent chromate film are generally expressed by xCr 2O3, yCrO3 and zH2O. The crystal water in this coating is vaporized when exposed to temperatures higher than 80°C, resulting in deterioration of the coating and poor resistance to corrosion. Therefore, care shall be taken. 4. QUALITY 4.1 Appearance The appearance of the plating shall be smooth and free from burn deposits, blisters or exposure of sub3 strate ( ) or any other defects detrimental to use. However, slight interference fringes or color irregularities, as a result of the chromate treatment, are acceptable. 3
Note ( ): Where the substrate contains defects that are not detected by a normal method, the defects may sometimes appear after plating as plating defects.
4.2 Plating thickness Plating thicknesses shall conform to Table 1. 4.3 Corrosion resistance of coating The corrosion resistance of the coating shall conform to Table 2 when tested by NES M 0140 (Methods of Salt Spray Testing).
3
M4040 [2006-N] Table 2 Type
Type 1
Type 2
Yellow chromate
Bright chromate
Type 3
Type 4
Type 5
Type 6
Black chromate
Olive chromate
Trivalent clear chromate
Trivalent black chromate
Corrosion resistance Time before any white corrosion material 4 appears ( ) (hours)
Time before any iron oxide (red rust) appears (hours)
Rating
Symbol
1
MFZn 3C
2
MFZn 5C
3
MFZn 8C
4
MFZn 13C
240
1
MFZn 3B
48
2
MFZn 5B
3
MFZn 8B
4
MFZn 13B
144
1
MFZn 3K
96
2
MFZn 5K
3
MFZn 8K
4
MFZn 13K
240
1
MFZn 3G
240
2
MFZn 5G
288
3
MFZn 8G
4
MFZn 13G
480
6
MFZn 25G
480 ( )
1
MFZn 3TC
96
2
MFZn 5TC
3
MFZn 8TC
4
MFZn 13TC
240
1
MFZn 3TK
96
2
MFZn 5TK
3
MFZn 8TK
4
MFZn 13TK
96 72
24
72
168
144 192
72 96
144 192
360 5
6
72 ( )
6
72 ( )
144 192
144 192 240
4
Note: ( ) The black material created before the white corrosion material appears will not be regarded as corrosion. 5 ( ) Time before any iron oxide (red rust) appears on bent parts such as brake tubes shall be 408 hours. 6 ( ) Slight whitening of the chromate film shall not be regarded as corrosion.
5. TESTS 5.1 Pretreatment of test sample The test part’s surface shall be cleaned to completely remove oils and stains and degreased using benzene, etc. 5.2 Appearance test The test shall be performed under bright light source (300 lx and over) when examining for burn deposits, blisters, exposed substrate, etc. 5.3 Thickness test As a general rule, a microscope should be used for measuring the p lating thickness. However, a coulometric thickness gauge or a magnetic thickness gauge may be used at the discretion of the personnel involved in the testing. However, if the coulometric thickness gauge is used, the results should be confirmed with a microscope as required. 5.3.1 Microscope method The sample to be measured for plating thickness shall be cut perpendicular to the plated surface. Then, the section shall be polished carefully and the plating thickness measured with the microscope. If the boundary between the substrate and the plating is not clearly visible, the test sample may be dipped for several seconds in a corrosion solution consisting of 5 m ℓ of non-diluted hydrochloric acid and 95 m ℓ of distilled water and then measured. Each test sample (one part) shall be measured at several locations and the minimum value shall be used as the thickness measurement of the sample.
4
M4040 [2006-N] 5.3.2 Plating thickness measurement using the coulometric thickness gauge The coulometric thickness gauge is used together with a microscope as a supplementary measuring method. Use the actual part as the test piece (no test piece cutting is required). Use the plated metal surface as the anode. Immerse the part in the electrolytic solution. Run a steady electric current through the electrolytic solution. Electrolysis will result. Initially, a constant voltage potential will appear between the anode and the cathode, corresponding to the metal used in the plated part. Immediately following this, complete dissolution of the metal plating on the part will occur and the base surface of the part will be clearly visible. Two rapid voltage fluctuations between the anode and the cathode will occur. Note the time of the second voltage fluctuation. Electrolysis is now complete. Plating thickness is determined based on the time required for complete electrolysis. 5.3.3 Plating thickness measurement using the magnetic thickness gauge (1) The magnetic thickness gauge is used whenever the actual part, which cannot be damaged, is used. (2) Use an actual part as the test piece. Measure the plating thickness on a flat and smooth area of the part as much as possible. 6. SALT SPRAY TEST The salt spray test shall be performed according to NES M 0140. Continuous spraying shall be applied for the specified time. 7. INSPECTIONS Plated parts shall be inspected for appearance, the thickness of plated film, and resistance to corrosion. (1) For sampling, many samples shall be extracted at random from lots of the same parts. The number of test samples and inspection positions shall be agreed upon between buyer and supplier. (2) The appearance of the plated film shall be inspected and tested according to Item 5, and the result shall conform to the specifications of Item 4. 8. PREVENTION OF HYDROGEN EMBRITTLEMENT (1) Removal of stress before plating The basis metals such as high-strength steel, which has high tensile strength, and case hardening steel shall be protected from hydrogen embrittlement. To reduce the effect of hydrogen embrittlement, heat treatment to remove stress before plating is desirable. The conditions shall be agreed upon between buyer and supplier. (2) Prevention of hydrogen embrittlement after plating When the steel parts have a hardness of 28 HRC (269 HB) or above or are highly cold-worked parts, the plated parts shall be protected from hydrogen embrittlement. To protect from or prevent hydrogen embrittlement, the pretreatment of plating and the plating work shall be adequately performed. For the parts using spring steel, etc., within 4 hours after plating, heat treatment shall be conducted to prevent hydrogen embrittlement. Heat treatment condition described in JIS is shown in Table 3 for reference. Table 3 Tensile strength (MPa)
Heat treatment condition
Removal of stress before plating Temperature (°C) Hour (h)
Prevention of hydrogen embrittlement after plating Temperature (°C) Hour (h)
Up to and incl. 1,050
―
―
―
―
1,051 to 1,450
190 to 220
1
190 to 220
8 Min.
1,451 to 1,800
190 to 220
8
190 to 220
18 Min.
Above 1,800
190 to 220
24
190 to 220
24 Min.
9. DRAWING NOTATION The number of the Standards and symbols shall be designated on the drawing. Example: Electroplated zinc coating Type 5 trivalent clear chromate grade 3 NES M 4040 —
MF
Zn
8
TC Trivalent clear chromate Plating thickness (minimum) Zinc Iron substrate Standard No.
5
